GLACIATION OF PRINCE WILLIAM SOUND 485
small piedmont bulbs as they do in the Yakutat Bay region, but we have seen no examples of it. In the Copper River valley, however, there are some excellent examples, notably Miles and Allen Glaciers, which, descending from tributary valleys, expand on the broad floor of the Copper River canyon into true piedmont bulbs.
Among the glaciers that we have studied in Fringe William Sound, only one or two bore an extensive burden of ablation moraine, such as one finds so commonly in the Yakutat Bay region. This may be due partly to the lesser friability of the rocks in the valleys through which the Prince William Sound glaciers flow, but it is partly due and perhaps mainly, to the fact that the glacier ends are not subjected to the long-continued wastage of the stagnant, piedmont condition. That the latter is the main reason is suggested by the fact that the piedmont bulbs of the Miles, Allen, and many other glaciers of the Copper River whose d6bris supply is from rocks of similar character to those of the Prince William Sound region are mantled with ablation moraine like that upon the glaciers of Yakutat Bay.
Causes of Recent Advances. As is fully stated in earlier chapters some of the glaciers of Yakutat Bay have undergone a remarkable, recent, spasmodic advance and transformation, which we believe to be the result of an impulse originating in sudden and unusual accession of snow in the glacier reservoirs during the severe shaking which accompanied the earthquakes of September, 1899. No similar recent advance and transformation was observed in the glaciers of Prince William Sound, but Childs Glacier on the lower Copper River had a similar episode in 1910.
The recent advance of the Columbia and 14 other Prince William Sound Glaciers is not in the same class with the advance of the 9 ice tongues of the Yakutat Bay region, either in rate of change or extent of the transformation. The recent advance of the Prince William Sound glaciers may have been a response merely to minor variations in climate; or it may be a response to some period of slight earthquake shaking in the Chugach Mountains. Upon this question it is at present impossible to speak with more definiteness, for we have a very limited series of observations of these glaciers. That the period of advance and recession, and that the extent and degree to which these variations take place, are different in the two regions is to be expected.
In order that the explanation of the variations may become established it is important that we have data covering a long period of years. At present our difficulty is the briefness of the period of observation, and the fact that we do not have a larger basis for comparison. When this work has covered a wider area and a greater number of years the Alaskan glaciers may be expected to give answers to some of the glacial problems not only of local, but of widespread and even general interest. Their activity and variability, their vastness and variety of form, lead one to expect important results as an outcome of connected study of their characteristics and behavior.